In some 'Harry Potter turns evil' fanfictions, the death of a loved one, like Sirius, could be the tipping point. Grief and anger could consume Harry. He might think that the light side of magic has failed him. So, he starts exploring the dark arts more deeply. He could also form alliances with Death Eaters who promise him power and the ability to change the wizarding world according to his new, darker vision. As he delves deeper into the dark side, his personality changes completely, becoming cold and calculating, unlike the kind Harry we know from the original books.
Well, perhaps in this fanfiction, Harry is constantly bullied at Hogwarts in a much more extreme way. This continuous abuse could warp his sense of right and wrong, making him seek revenge on those who have wronged him. He might start using dark magic to get back at his tormentors and gradually slide into evil. He could also lose faith in Dumbledore's leadership and view of the world, which could be a major factor in his turn to the dark side.
One possible plotline could be that Harry discovers a dark secret about his family and feels betrayed by the wizarding world. This could lead him to side with the dark forces. Another could be that he is influenced by a powerful dark wizard who manipulates his mind. Maybe he becomes envious of the power others hold and decides to take it by any means, turning evil in the process.
Maybe Harry discovers some dark family secrets that make him question his place on the 'good' side. He could find out that his ancestors had strong ties to dark magic, and this knowledge could slowly corrupt him. He might start to experiment with dark spells, thinking that he can control them and use them for what he believes is the greater good, but gradually slips towards the darkside.
One possible plotline could be that Voldemort rescues Harry because he sees potential in him to be a powerful ally in his quest for total domination. Harry, being evil, might have already started to dabble in the Dark Arts on his own and Voldemort wants to harness that. Another plotline could be that Voldemort saves Harry from some other threat, like the Ministry of Magic which has turned against Harry for his evil deeds. This could lead to a strange alliance between them as they both have a common enemy in the Ministry.
One possible plotline could be that Lily, now as a girl, has to navigate the wizarding world all over again. She might face different challenges and opportunities than she did before. For example, she could form new friendships or rivalries at Hogwarts.
One possible plotline could be that the Veela is drawn to Harry's new - found evil side, perhaps seeing power in it. And Harry uses the Veela's charm and abilities for his evil plans, like luring enemies or getting information. Another could be that the Veela tries to redeem Harry from his evil ways, creating a sort of internal struggle within Harry between his evil intentions and the Veela's positive influence.
One possible plotline could be that Harry, in his 'evil' form, forms an alliance with Fenrir. They might decide to take over the wizarding world together, using Fenrir's werewolf strength and Harry's knowledge of magic. Another could be that they are on a quest to find a powerful magical artifact that can change the fate of the werewolves in the wizarding world.
One plotline could be Naruto being a transfer student at Hogwarts. He might struggle to adapt to the magic system while also showing off his ninja skills. Another could be Naruto teaming up with Harry Potter to fight a common enemy that threatens both the ninja world and the wizarding world.
One possible plotline could be Harry using his knowledge of the wizarding laws, which he learned through his years at Hogwarts and his experiences with the Ministry of Magic, to defend wrongly accused wizards in a wizarding court. Another could be him trying to reform the wizarding legal system which he might find full of loopholes and biases.
One possible plotline could be Harry trying to hide his merman identity while at Hogwarts. He might face challenges in the magical underwater areas of the school, like the lake. Another could be that he discovers his merman heritage and has to go on a journey to learn more about it and his new powers.
One possible plotline could be Harry being secretly trained by an unknown figure to become an assassin against the dark forces. He would have to hide this new identity from his friends at Hogwarts.
One possible plotline could be about the Weasley family trying to improve their financial situation. Maybe they come up with a new business idea in the wizarding world, like a magical product that becomes a huge success and changes their economic status. Another could be a story about how the Ministry of Magic regulates the financial system in the wizarding world, and there are some characters who try to manipulate or reform it.